Flight of the Phoenix

Building a ‘floating’ sixth form college in Shepherd’s Bush, West London, required an advanced hybrid steel and concrete frame and intricate installation. Stephen Cousins reports. Photographs by Ben Clarkson

The new £9m Phoenix Sixth Form Centre in Shepherd’s Bush, West London

“Standing here with this huge weight above you is perturbing. In your own head you can’t work it out, it’s like your brain is trying to tell you the whole thing’s about to fall on you, even though you know it can’t,” says Kevin Rogers, contracts manager at main contractor Bowmer & Kirkland, in a slightly worrying turn of phrase. He’s showing me around the site of the new £9m Phoenix Sixth Form Centre in Shepherd’s Bush, West London, which is due to welcome its first students this summer.

Above us, a huge, 1,100-tonne, two-storey cantilevered concrete classroom block hangs in mid air. Roughly 24 metres long and supported by a pair of thin steel columns that form a V shape at the far end, the cantilevered section covers around twice the area of the podium block beneath.

Even though I know Rogers isn’t serious about the threat of collapse, there’s still enough concrete overhead for me to give silent thanks for the skills of the engineer and contractor. “It’s like a plane in flight, you think ‘how does such a heavy object stay in the air?’” Rogers adds, as if reading my thoughts.

This gravity-defying propped cantilever was made possible thanks to an ingenious hybrid structural system, devised by structural engineers SKM Anthony Hunt and Bowmer & Kirkland, which combines a super-reinforced concrete frame with deep foundation piles, two structural steel columns, and tensioned steel bars. Designed to withstand disproportionate collapse — where the failure in one structural element has a knock-on effect elsewhere — the elements work in unison to support the cantilever, as well as prevent it from twisting horizontally, or vertically wrenching the rest of the building from the ground.

The majority of the cantilever comprises reinforced concrete walls, poured in-situ, and lightweight pre-cast Omnicore plank floor slabs.

A complicated load path transfers some of the forces generated by the cantilevered section through two diagonal steel tensioned hangers, supplied by Macalloy, sited just behind the glazed wall at the end of the cantilever. This load is in turn transferred into the two central corridor walls, positioned on concrete beams, which channel the resulting 15,000kN load into the steel columns below.

What’s even more impressive is that this complex structural solution was arrived at via a multi-disciplinary redesign carried out in just six weeks. The original design, by architect Bond Bryan and SKM Anthony Hunt, was for a pure cantilever with no support columns. Bowmer & Kirkland was appointed main contractor under a £7.3m design and build contract in January 2010, but just days after groundworks had begun in February, technical issues forced a rethink of the design and construction was put on hold. Bowmer & Kirkland was then asked by the London Borough of Hammersmith & Fulham to manage the redesign under its contract, while simultaneously rewriting the budget and programme.

“Our immediate concern was how the redesign would effect the scheme as a whole, particularly at that stage in the project,” says Rogers. “We quickly sat down with the school’s team and agreed a plan to minimise the effects before the deadline for return to site. Timing was everything: if the six-week period had overrun, resulting in project handover [for client fit-out] not being achieved by March 2011, then the project may never have gone ahead.”

The cantilevered concrete classroom block is supported by a V-shaped column, with Macalloy hangars shown front

Brightly coloured fins conceal ventilation ducts and provide solar shading

The building pivots over the podium

Wary of a poor structural compromise, SKM Anthony Hunt worked hard to devise a propped cantilever design with minimum support, says technical director Chris Kitching. “We couldn’t live with a sea of columns, so we started strategically taking them out one by one. However, as you remove columns, the structural solutions become very different. We ended up with a hybrid composite structural frame, which is arguably even more impressive because it’s so complex.”

The Phoenix Sixth Form Centre is the brainchild of outspoken head teacher Sir William Atkinson, who was knighted in 2008 after turning round the former Hammersmith School, which had a reputation as Britain’s worst-performing school. Renamed Phoenix High School, Atkinson lifted it from a 5% A-C GCSE pass rate to a level in line with the national average.

Atkinson now has similarly ambitious plans for the new sixth-form building, which stands on a site next to the school. His brief called for an iconic design that would inspire 16-year-old pupils to stay on and complete their A-levels.

Architect Bond Bryan sets out to challenge the unremarkable local context and create a real sense of spectacle. Approaching the building through the area’s run-down housing estates, the first thing you notice is the colours — primary blue, red, yellow and green adorning the sides of nine timber fins that project and twist from the facades at raking angles. More than just an eye-catching visual device, the fins conceal ventilation ducts and services, and provide solar shading for the windows in between.

Space and security considerations drove the decision to elevate half of the building above the existing car park, explains Jeff Stibbons, project director at Bond Bryan. “Staff were adamant that on-site car parking be retained for their security, but building a new facility would have meant digging up the school playing fields or other ecologically valuable land, so we went for a cantilevered solution, which also helped preserve views from the adjacent classroom block,” he says.

At the northern end of the building a double-height oriel window encloses the learning resource centre (LRC) and projects over a playing field below. With this structure at one end and the propped cantilever at the other, the building appears to pivot over the two-storey concrete podium block like a see-saw.

This concrete block covers about half of the footprint of the building, and houses a two-storey glazed entrance atrium, a kitchen and open-plan dining facilities on the ground floor, with IT suites and classrooms on the first floor. Inside the atrium, brightly clad blob-shaped hub spaces provide private study areas.

The second and third floors — in the cantilevered section — accommodate maths and English classrooms, chemistry and biology laboratories, staff rooms and the head teacher’s office, all arranged along a central corridor that extends the length of the building and provides crucial structural support.

At the northern end of the building double-height glazing encloses the learning resource centre

A forest of props was needed to support the cast in-situ frame and cantilever

The new Sixth Form Centre sits on a tightly enclosed site

The weight of the cantilever also creates tensile forces in the podium block. To prevent cracking or see-sawing, large amounts of concrete reinforcement, laid out in diagonal arrangements, was embedded inside the cantilever’s 400mm-thick external walls, while piles embedded up to 25 metres deep underneath the block further anchor the block to the ground. A secondary steel structure was fixed to the concrete walls to attach the cedar and coloured Trespa cladding of the projecting fins.

“Most reinforced concrete buildings only deal with compressive loads, but here the magnitude of the forces was so high there was tension in some elements, and the only way to deal with it was to use extra reinforcement. If this had not worked, any cracking [during the concrete pour] would have meant significantly altering the design,” says SKM’s Kitching.

Changes to the location of the concrete rebar during the redesign also forced SKM, Bond Bryan and the M&E contractor, Derry Building Services, to re-plan the locations of windows and service penetrations through the elevations to avoid clashes with rebar or downstand beams.

The construction sequence required to create the building’s concrete box, planned with concrete subcontractor John Doyle Construction, was also demanding. Thousands of props were required to support the frame and cantilever, which had to remain in place until the entire structure had sufficiently “cured” to the correct strength. The concrete was poured in sequence from the north LRC end towards the south because the north end foundations would provide the necessary tension to support the cantilever. 

The extent of the propping was unusual for a project of this type, explains Rogers: “On a typical cast in-situ concrete frame you would strike the propping as each section cured, but here the nature of the structure meant we had to wait for the entire building to reach 70-80% of design strength before we could start to strike. Under the cantilever there was an absolute forest of props, you could hardly walk through it. It sterilised the site for over 22 weeks.”

The props were removed one at a time under a strictly-timed sequence, as SKM’s engineers closely monitored forces in the structure and simultaneously brought the Macalloy steel hangers into tension to take the load. If the sequence or stresses had been misjudged, there was a genuine threat of collapse, says Rogers.

Fortunately for the project’s future students and visiting journalists, that didn’t happen. Thanks to close co-ordination between designers and contractor, the Phoenix’s cantilever is now in full flight. 

Macalloy system delivers stress relief

Phoenix Sixth Form’s two-storey propped cantilever walls were brought into tension using an adjustable steel rod system at the end of the cantilever.

Manufactured by Macalloy, the system comprises two pairs of steel rods with forked metal ends, which are embedded into concrete at roof level on either side of the cantilever’s central corridor and span diagonally out to the base of the cantilever walls on the external facade.

Each bar is fitted with the manufacturer’s patented TechnoTensioner system, a type of turnbuckle with threads at either end of the bolt adjusted using a torque wrench. When tensioned, and after propping has been removed from under the cantilever, the bars help prevent short- and long-term vertical deflections.

Before tensioning could begin, structural engineer SKM Anthony Hunt had to calculate the as-constructed total load on the building, including any secondary steelwork, fittings and finishes, and construction live load. Based on this the engineer determined target stresses for each bar.

Then, as propping was gradually struck, and under SKM Anthony Hunt’s supervision, concrete subcontractor John Doyle Construction individually tensioned each of the four bars, which allowed for fine adjustment and easier measurement of forces within the system. Once the falsework was removed the system was able to achieve target tension values of 1,080kN per bar, or 2,160kN per pair.

“Everything had to be spot on or there was a real threat of collapse,” says Kevin Rogers, contracts manager at Bowmer & Kirkland.